Varactor Shunt Research Articles

Constant absolute bandwidth six-pole quasi-elliptic tunable bandpass filter using synchronously tuned dual-mode resonators

This paper presents the design of a 6th-order tunable bandpass filter (BPF) that achieves constant absolute bandwidth (ABW) and transmission zeros (TZs) across a wide frequency tuning range. The proposed tunable BPF employs three synchronously tuned dual-mode resonators, along with a frequency-dependent coupling structure realized using a T-network consisting of series and shunt capacitors. To achieve constant ABW, the tunable external quality factors are implemented using series capacitor and shunt varactor. As an illustrative example, a 6th-order BPF is fabricated and measured, results revealing that the center frequency of BPF can be tuned from 1.39 GHz to 1.81 GHz (26.25 %) with two TZs on the lower and upper side of the passband, 1-dB constant ABW of 62 ± 1.5 MHz and return loss > 12 dB in every tuning state.

A Self-Matching Rectifier Based on an Artificial Transmission Line for Enhanced Dynamic Range

In this paper, self-matching rectifiers for enhanced input dynamic range are demonstrated. The method is based on an artificial transmission line, which consists of a stepped-impedance transmission line loaded with shunt varactors. The output DC voltage of the rectifier is fed back to the varactors to control the effective electrical length and characteristic impedance of the line. Because this bias voltage changes as the power input to the rectifier changes, a matching network that maintains impedance matching regardless of the input power can be developed without additional circuits, thus substantially improving the input dynamic range. The experimental results for prototype rectifiers show a dynamic range as wide as 25 dB with only a single Schottky diode, an improvement of 11 dB over the rectifier with a conventional matching scheme, in which the conversion efficiency remains above 50%. The effectiveness of the proposed method is also verified with the experimental results for rectennas based on the demonstrated rectifiers.

3-D-Printed Tunable Circularly Polarized Microstrip Patch Antenna

We present a tunable circularly polarized (CP) microstrip patch antenna developed using three-dimensional (3-D) printing. Circular polarization is achieved by a novel method of introducing an L-shaped slot in a square patch antenna. Moreover, by loading the four edges of the patch with shunt varactors, the frequency of the antenna can be tuned while maintaining CP radiation. The use of 3-D printing for manufacturing enables the vertical integration of varactors between the patch and the ground, which has not been shown before. Such integration minimizes the interaction of varactors with the radiating properties of the antenna and results in reduced parasitics. Measured results show that the antenna has a frequency tuning range of 27.2% for CP operation with the center of tuning range around 1.9 GHz. The 3 dB axial-ratio bandwidth remains above 3% for each fixed frequency band of operation.

Bandwidth‐tunable filtering balun based on compact 3D configuration

For the first time, a novel bandwidth-tunable filtering balun based on compact 3D configuration is presented. The overall design is basically composed of a two-layer horizontal feeding plane, a three-layer vertically mounted plane, and the transmission-line circuits disposed on both. Implementing the ring resonator in vertical double-sided parallel-strip line, the complete horizontal ground and frequency-independent phase difference are subtly combined. Connecting the shunt varactor diodes with the inserted vertical ground, the desirable filtering property and flexible bandwidth tunability are splendidly fulfilled. A prototype balun centred at 1.53 GHz is fabricated. The measured operating bandwidth is dynamically tuned from 0.87 to 1.23 GHz with a return loss <−10 dB, forcefully validating the design concept.

Frequency‐agile ring resonator end‐loaded slot antenna

AbstractIn this letter, a novel frequency‐agile Ring Resonator End‐Loaded Slot Antenna is proposed, analyzed with a transmission line model, and experimentally verified. The innovative end‐loads are annular slot ring resonators made tunable by way of shunt varactor diode loading. The antenna covers most common mobile communication bands with continuous frequency agility between 1.65 and 2.9 GHz (55% tuning range)—providing realized gains of 2 to 6.6 dB and radiation efficiency on the order of 50% to 90%, respectively. This leads to an improvement of nearly 10 dB in signal‐to‐noise‐ratio in a line‐of‐sight communication link over previously reported frequency agile antennas. The radiation patterns follow the shape of a straight slot radiator on a finite ground plane and vary only slightly when the antenna is tuned to different frequencies.

A New Passive CMOS Mixer With LO Shaping Technique for mmWave Applications

A mmWave (mmW) passive mixer topology based on a novel LO waveform-shaping technique using non-linear transmission line (NLTL) is proposed. The NLTL is constructed using series inductors and shunt varactors. It boosts the LO signal swing and reshapes the LO waveform to have higher amplitude and narrower pulses, resulting in higher gain, better linearity, and lower Noise Figure (NF) of the mixer. Mixers operating at 60 GHz are designed and implemented using 65 nm CMOS process. Simulation results show the mixer with the proposed technique obtains an NF of 5.5 dB, an input-referred $\text{P}_{1 \text{dB}}$ $(\text{IP}_{1 \text{dB}})$ of 0 dBm, and a voltage conversion gain of 1.2 dB with 0 dBm LO signal and zero dc power consumption. Silicon measurement results show that the proposed LO shaping technique improves the conversion gain by 2.5 dB and the $\text{IP}_{1 \text{dB}}$ by 3 dB.

Calibration Technique for Maximum Power Harvest of Passive Wireless Microsystems

A calibration technique for maximizing radio-frequency power harvest of passive wireless microsystems with a step-up transformer is proposed. We show that both the impedance and resonant frequency of the step-up transformer matching network can be adjusted by varying the capacitance of a shunt varactor placed at the secondary winding of the transformer to maximize power transfer from the antenna to the transformer and the output voltage of the transformer subsequently the power efficiency of the voltage multiplier. A low-power current-mode tuning technique and a maximum peak amplitude detection technique to allocate the optimal tuning capacitance at which the maximum power harvest exists are introduced. The transformer matching network has been designed in IBM CMRF8SF 130-nm 1.2-V CMOS technology, and its performance is validated using both simulation and on-wafer measurement results.

Dual C- and S-Band CMOS VCO Using the Shunt Varactor Switch

This paper proposes a high-performance CMOS voltage-controlled oscillator (VCO) with two subfrequency bands. The VCO consists of two cross-coupled VCOs coupled by a pair of mode-switched inductors. Two pairs of shunt varactors are used to switch high- and low-frequency bands. With the varactors as the mode switches, the odd mode VCO operates at the high frequency (C-band) and the even mode VCO operates at the low frequency (S-band). The proposed VCO has been implemented with the TSMC 0.18- $\mu $ m 1P6M CMOS technology and it can generate differential signals in the frequency range of 5.21–6.66 GHz (IEEE 802.11a) and 3.26–3.84 GHz (IEEE 802.16a) and it also has high output voltage swings at both low- and high-frequency bands. The die area of the dual-band VCO is 0.976 mm $\times 1.092$ mm. At the supply voltage of 0.75 V, the high(low)-band figure of merit is −190.5(−190.4) dBc/Hz.

A Reconfigurable Coplanar Waveguide Bowtie Antenna Using an Integrated Ferroelectric Thin-Film Varactor

A novel printed antenna with a frequency reconfigurable feed network is presented. The antenna consists of a bowtie structure patch radiating element in the inner space of an annulus that is on a nongrounded substrate with a ferroelectric (FE) Barium Strontium Titanate (BST) thin film. The bowtie patch is fed by a coplanar waveguide (CPW) transmission line that also includes a CPW-based BST shunt varactor. Reconfiguration of the compact 8 mm × 8 mm system has been demonstrated by shifting the antenna system’s operating frequency 500 MHz in the 7–9 GHz band by applying a DC voltage bias.

SUBSTRATE INTEGRATED WAVEGUIDE FREQUENCY-AGILE SLOT ANTENNA AND ITS MULTIBEAM APPLICATION

A new technique for designing the substrate integrated waveguide (SIW) frequency-agile slot antenna is presented in this paper. Similar to the metallic waveguide counterpart, the SIW is a uniconductor guided-wave structure and inherently di-cult in the electronically tunable applications. To solve this problem, a single slot etched on the top conductor layer of a conventional antenna is instead of two slots to construct an isolated area, which is convenient for the DC bias. The electric length of slots can then be adjusted through two shunt varactors welded near the center of slot. As such, a SIW frequency-agile slot antenna is realized and fabricated to cover the frequency of 2:30 » 2:74GHz with difierent bias voltages. Experiments verify our theory analysis and design process. Then, a frequency-agile multibeam antenna is developed to cover several frequency bands while switching between four difierent radiation patterns pointing at difierent spatial locations for each frequency. It presents an excellent candidate for software and cognitive radio system applications.

Reconfigurable Small-Aperture Evanescent Waveguide Antenna

We present a reconfigurable small-aperture evanescent wave guide antenna. Tuning and waveguide matching to free space are simultaneously achieved by placing a printed iris with a shunt varactor diode connected across the aperture of a below-cutoff waveguide. A design approach using the Imaginary Smith Chart is used to synthesize the antenna. The maximum dimension of the antenna is 0.36λ at 2.49 GHz and it operates over an operating frequency range of 2.05-2.49 GHz with maximum realized gain of 5 dBi. The antenna-due to its small size and compact feed arrangement-can be used in phased array/cognitive radio applications. Also its tuneable return loss characteristics enable it to act as an electro magnetic switch (shutter), or variable RCS aperture.

Fixed-Beam Frequency-Tunable Phase-Reversal Coplanar Stripline Antenna Array

A fixed-beam frequency-tunable coplanar stripline (CPS) antenna array using the phase reversal technique and varactor diodes tuning is presented. The antenna array is composed of plurality of half-wavelength CPS sections interconnected by phase reversing crossovers. These crossovers have two functions: they serve as the radiators (small dipoles) of the array and also provide their own required in-phase excitations for broadside radiation. Two microstrip-to-CPS transitions for excitation of the balanced input/output antenna array are implemented and compared. A six-element array is demonstrated theoretically and experimentally, with 9.3 dBi of gain for bidirectional radiation and 12.5 dBi of gain for unidirectional radiation using a back reflector. Moreover, shunt varactor diodes are incorporated along the CPS structure to achieve frequency tuning through a bias voltage control while broadside radiation patterns remain fixed. The return loss and the radiation patterns at several frequencies are presented, and up to 50% tuning range is obtained. The proposed antenna array is simple and uniplanar with small lateral size, high radiation efficiency and high directivity. In addition, it uses a very simple biasing circuit with high DC-RF isolation. Its balanced input/output makes it suitable for RF system integration and active integrated antenna design.

BRIGHT AND DARK SOLITON GENERATION IN A LEFT-HANDED NONLINEAR TRANSMISSION LINE WITH SERIES NONLINEAR CAPACITORS

In this article, we demonstrate that in the case of a positive group velocity left-handed nonlinear (LH-NL) transmission line with series nonlinear capacitances, the spatial derivative of the voltage distribution satisfles the nonlinear Schrodinger (NLS) equation. Consequently, it will shown that a LH-NL transmission line with series varactors can be used to generate both bright and dark solitons similar to a composite right-left-handed (CRLH) transmission line periodically loaded with shunt varactors. The paper also discusses the conditions for generation of bright and dark solitons.

Metamaterial-based electronically controlled transmission-line structure as a novel leaky-wave antenna with tunable radiation angle and beamwidth

A metamaterial-based electronically controlled transmission-line structure is presented and demonstrated as a novel leaky-wave (LW) antenna with tunable radiation angle and beamwidth functionalities. This structure is, in essence, a composite right/left-handed (CRLH) microstrip structure incorporating varactor diodes for fixed-frequency voltage-controlled operation. Angle scanning at a fixed frequency is achieved by modulating the capacitances of the structure by adjusting the (uniform) bias voltage applied to the varactors. Beamwidth tuning is obtained by making the structure nonuniform by application of a nonuniform bias voltage distribution of the varactors. A rigorous analysis based on an extension of the CRLH concept is proposed and the corresponding dispersion curves, obtained by equivalent-circuit formulas with LC parameters extracted from full-wave simulation, are shown. A 30-cell LW antenna structure, incorporating both series and shunt varactors for optimal impedance matching and maximal tuning range, is designed. This prototype exhibits continuous scanning capability from 50/spl deg/ to -49/spl deg/ by tuning the bias voltages from 0 to 21 V at 3.33 GHz. A maximum gain of 18 dBi at broadside is also achieved. In addition, it provides half-power beamwidth variation of up to 200% with comparison to the case of uniform biasing. The effect of intermodulation due to the nonlinearity of the varactors is shown to be negligible for antenna applications. The antenna is tested in a 10-Mb/s binary phase-shift keying transmission link and successful recovery of the baseband data is demonstrated.

Metamaterial-Based Electronically Controlled Transmission-Line Structure as a Novel Leaky-Wave Antenna With Tunable Radiation Angle and Beamwidth

A metamaterial-based electronically controlled transmission-line structure is presented and demonstrated as a novel leaky-wave (LW) antenna with tunable radiation angle and beamwidth functionalities. This structure is, in essence, a composite right/left-handed (CRLH) microstrip structure incorporating varactor diodes for fixed-frequency voltage-controlled operation. Angle scanning at a fixed frequency is achieved by modulating the capacitances of the structure by adjusting the (uniform) bias voltage applied to the varactors. Beamwidth tuning is obtained by making the structure nonuniform by application of a nonuniform bias voltage distribution of the varactors. A rigorous analysis based on an extension of the CRLH concept is proposed and the corresponding dispersion curves, obtained by equivalent-circuit formulas with LC parameters extracted from full-wave simulation, are shown. A 30-cell LW antenna structure, incorporating both series and shunt varactors for optimal impedance matching and maximal tuning range, is designed. This prototype exhibits continuous scanning capability from 50/spl deg/ to -49/spl deg/ by tuning the bias voltages from 0 to 21 V at 3.33 GHz. A maximum gain of 18 dBi at broadside is also achieved. In addition, it provides half-power beamwidth variation of up to 200% with comparison to the case of uniform biasing. The effect of intermodulation due to the nonlinearity of the varactors is shown to be negligible for antenna applications. The antenna is tested in a 10-Mb/s binary phase-shift keying transmission link and successful recovery of the baseband data is demonstrated.

The distributed RC variable-frequency phase-shift oscillator

Methods of realising the voltage-controlled variable-frequency distributed RC phase-shift oscillator in integrated form are considered. A metal-oxide-semiconductor capacitor structure in which the metal is a nichrome resistor is specified. Two methods of varying the oscillation frequency are considered. The first uses the voltage-variable capacitance of the MOS structure. A simplified theory predicts the transfer characteristics well. The available frequency range as an oscillator is small because of the limited capacitance range of the devices used. However a frequency-voltage characteristic linear to better than 0.05% from 4.85 MHz to 5.1 MHz is observed. A very much wider frequency range is expected with improved devices. A second method of varying oscillation frequency uses a shunt varactor diode. The theory presented gives good agreement with measurements of transfer characteristics and predicts an attenuation near to 21 dB at 180° phase shift over a useful frequency range. This is also confirmed experimentally. The performance as an oscillator is limited in frequency by the non-ideal amplifiers used.